Purified fibrinogen was exposed to a limited action of thrombin, further thrombin activity being prevented by hirudin. Agarose gel chromatography of the soluble material revealed formation of derivatives of higher molecular weight than that of the parent fibrinogen molecule. When 125I-fibrinogen was added just before chromatography, a great proportion of the radioactivity was eluted together with the high molecular weight derivatives (HMWD), indicating copolymerization of fibrin monomer and fibrinogen. In contrast, 125I-FDP, consisting mainly of fragments X and Y, were only to a small extent eluted with HMWD, suggesting that early FDP are less capable of linking with fibrin monomer, than is fibrinogen. The ability of early FDP to link with fibrin was markedly increased, however, upon treatment with thrombin. Thus, early FDP copolymerize with fibrin monomer provided that fibrinopeptide A, known to be retained, is released.